1. Field of the Invention
The invention relates to the identification, purification, and isolation of a novel protein which interacts with Bcl-2 protein to form heteromultimers (heterodimers) in vivo and more particularly to the purification, isolation and use of Bcl-2 associated protein, herein called Bax.
2. Description of the Related Art
Cell death is an important aspect during the embryonic or post-natal development of major organ systems. Apoptosis, or programmed cell demise, also plays a critical role in maintaining homeostasis in many adult tissues. Within vertebrates, Bcl-2 is the best understood gene in a cell death pathway and functions as a cell death repressor.
Bcl-2 is unique among protooncogenes by being localized to the mitochondrial membrane as defined by Hockenbery, D. M., Nunez, G., Milliman, C., Schreiber, R. D. and Korsmeyer, S. J. "Bcl-2 is an inner mitochondrial membrane protein that blocks programmed cell death." Nature 378, 334-336, 1990. Bcl-2 has been shown to have the oncogenic function of blocking programmed cell death whereas a deregulated Bcl-2 extends the survival of certain hematopoietic cell lines following growth factor deprivation. When pro-B-cell or promyelocyte cell lines are deprived of interleukin 3 they normally succumb to a programmed demise entitled apoptosis. This pattern of morphologic cell death is characterized by a dramatic plasma membrane blebbing, cell volume contraction, nuclear pyknosis, and internucleosomal DNA degradation following the activation of an endonuclease. Over expression of mitochondrial Bcl-2 appears to function as an antidote to this process and has the unique function of blocking programmed cell death independent of promoting proliferation.
The Bcl-2 protooncogene was discovered at the chromosomal breakpoint of the t(14;18) (q32;q21) which is the cytogenetic hallmark of human follicular lymphoma. Approximately 85% of follicular and 20% of diffuse B-cell lymphomas possess this translocation. Follicular lymphoma is often present as a low-grade malignancy composed of small resting IgM/IgD B cells. Over time, conversion to a more aggressive high-grade lymphoma with a diffuse large-cell architecture frequently occurs.
Studies of Bcl-2 emphasizes the existence of multiple pathways in the generation of neoplasia. The increased cell number in neoplastic tissue can be viewed as a violation of normal homeostasis. The maintenance of homeostasis in normal tissue, in many respects, reflects a simple balanced equation of input (cellular proliferation and renewal) versus output (cell death). This is most easily envisioned for encapsulated organs, such as the prostate, but is also true of the recirculating hematopoietic lineages. The maintenance of remarkably invariant cell numbers reflects tightly regulated death pathways as well as controlled proliferation. See for example S. J. Korsmeyer "Bcl-2 Initiates a New Category of Oncogenes: Regulators of Cell Death", Blood Vol. 80 No. 4 pp. 879-886, Aug. 15, 1992.
Programmed cell death represents a cell autonomous suicide pathway that helps restrict cell numbers. The well-defined loss of specific cells is crucial during embryonic development as part of organogenesis. In mature tissues, genetically programmed demise regulates the volume of cells. A morphologically distinct and temporally regulated cell death entitled apoptosis has been identified by Wyllie A H: "Apoptosis; Cell death in tissue regulation". J. Pathol 153:313, 1987. Cells dying by apoptosis display marked plasma membrane blebbing, volume contraction, nuclear condensation, and the activation of an endonuclease that cleaves DNA into nucleosomal length fragments.
Bcl-2 has been localized to chromosome segment 18q21.3 in a telomere to centromere orientation. The Bcl-2 gene possesses 3 exons, the first of which is untranslated. Two potential promoter regions exist. P1 is GC rich with multiple SP1 sites and is used predominantly. Bcl-2 is an enormous gene in which a 225-kb intron II divides the protein encoding exons II and III. See Silvermann G A et al. "Meiotic recombination between yeast artificial chromosomes yields a single clone containing the entire Bcl-2 proto-oncogene" Proc Natl Acad Sci 87;9913, 1990. A molecular consequence of the translation is the movement of the Bcl-2 gene to the der(14) chromosome placing Bcl-2 in the same transcriptional orientation as the Ig heavy chain locus giving rise to chimeric RNAs. However, translocation does not interrupt the protein encoding region so that normal and translocated alleles produce the same sized, 25-Kd protein.
Hematopoietic progenitors, including pro-B cells, possess high levels of Bcl-2. See Hockenbery D, Zuter M, Hickey W, Nahm M, Korsmeyer S J: "Bcl-2 protein is topographically restricted in tissues characterized by apoptotic cell death". Proc Natl Acad Sci U.S.A. 88:6961, 1991. Some mature B cells and, especially, B-cell lines have low levels of Bcl-2 RNA. In contrast, t(14;18)-bearing B cells have inappropriate elevated levels of the Bcl-2-Ig fusion RNA. Graninger W B, Seto M. Boutain B, Goldman, P, Korsmeyer S J: Expression of Bcl-2 and Bcl-2-Ig fusion transcripts in normal and neoplastic cells. J. Clin Invest 80:1512, 1987. This increased steady-state RNA reflects both increased transcription as well as a processing advantage for the Bcl-2-Ig fusion allele.
Bcl-2 has been introduced into a variety of interleukin (IL)-dependent cell lines to determine if it is involved in a growth factor pathway. See SJ Korsmeyer above. Such lines were examined to determine if Bcl-2 would spare the need for a specific ligand/receptor interaction. However, no long-term growth factor-independent cell lines emerged after overexpression of Bcl-2 in IL-2, IL-3, IL-4, or IL-6 requiring lines. However, Bcl-2 conferred a death-sparing effect to certain hematopoietic cell lines after growth factor withdrawal in the IL-3-dependent early hematopoietic cell lines FDCP1, FL5.12, and 32D. This effect was not restricted to the IL-3/IL-3 receptor signal transduction pathway in that granulocyte-macrophage colony-stimulating factor (GM-CSF) and IL-4 deprived cells displayed a similar response. Yet, Bcl-2 enhanced cell survival was not universal, as neither IL-2-dependent T-cell lines nor an IL-6-dependent myeloma line showed a consistent effect upon factor withdrawal.
Bcl-2 has not been shown to directly promote cell cycle progression, nor does it necessarily alter the dose response to limiting concentrations of IL-3. See Nunez G, London L., Hockenbery D, Alexander M, McKearn J, Korsmeyer S J: "Deregulated Bcl-2 gene expression selectively prolongs survival of growth factor-deprived hemopoietic cell lines". J. Immunol 144;3602, 1990. Instead, Bcl-2 blocked the plasma membrane blebbing, volume contraction, nuclear condensation, and endonucleolytic cleavage of DNA known as apoptosis. Factor deprived cells return to Go, but do not die. However, they can be rescued after 30 days of deprivation by the addition of IL-3, indicating they are not terminally differentiated or permanently arrested.
While identifying the Bcl-2 cell death pathway is significant, a way of regulating the Bcl-2 pathway has not been discovered. The ability to down-regulate the effect of Bcl-2 would be advantageous in cancer therapy, in controlling hyperplasias such as benign prostatic hypertrophy (BPH) and eliminating self reactive clones in autoimmunity by favoring death effector molecules. Up-regulating the effect of Bcl-2 and favoring death repressor molecules would be beneficial in the treatment and diagnosis of immunodeficiency diseases, including AIDs, and in neurodegenerative and ischemic cell death.